# merge_sort(nums,0,3)
#     mid = 1
#     merge_sort(nums,0,1)
#     merge_sort(nums, 2, 3)
#     merge(nums, 0, 1, 3)
#
# merge_sort(nums,0,1)
#     mid = 0
#     merge_sort(nums, 0, 0)
#     merge_sort(nums,1, 1)
#     # 合并阶段
#     merge(nums, 0, 0, 1)
#
# merge_sort(nums, 2, 3)
#     mid = 2
#     merge_sort(nums, 2, 2)
#     merge_sort(nums, 3, 3)
#     merge(nums, 2, 2, 3)

# 分的结论:
#  先 merge(nums, 0, 0, 1)
#  再 merge(nums, 2, 2, 3)
#  最后: merge(nums, 0, 1, 3)

# merge(nums, 0, 0, 1) 对 7 3 进行排序